The invention generally relates to a method of encapsulating components based on organic semiconductors. Preferably, it relates especially to organic light-emitting diodes, in which a housing is bonded adhesively to a substrate.
Components whose functioning is based on the use of organic semiconductors must be given effective protection against ambient influences, and especially against degradation caused by air or moisture. This purpose is generally served by a housing, which may also, for example, be a flat cover.
Components based on organic semiconductors are, in particular, organic light-emitting diodes (OLEDs). Light-emitting diodes of this kind feature organic monomers or polymers arranged between electrodes, with one electrode being transparent. When a voltage is applied to the electrodes, light is emitted. For this purpose OLEDs typically feature an organic electroluminescent material (emitter), an organic hole transport material, and an organic electron transport material. These materials, and the cathode material too, require protection against degradation caused by air (oxygen) and water, for which efficient capsuling is needed.
The capsuling of OLEDs can take place in a variety of ways: for example, by means of glass solders, i.e., by joining (glass) parts using a glass solder (German patent application file ref. 198 45 075.3). Already known as well is an electroluminescent system wherein the electroluminescent element is enclosed in a housing composed of a substrate, which carries the electroluminescent element, and a cover layer, made of a low-melting metal or a corresponding alloy and provided with an electrically conducting adhesive film (WO 97/46052). A disadvantage here is that the relatively high temperatures needed for processing the glass solder and/or the metal or alloy from the melt may result in damage to the electroluminescent element.
An organic electroluminescent device with a flat cover is known from EP 0 468 440 B 1. Atop the cathode in this device is a protective layer composed of a mixture of at least one component of the organic electroluminescent medium and at least one metal having a work function in the range from 4 to 4.5 Ev. The protective layer is produced by coevaporation of the aforementioned materials, although this involves a great deal of complexity.
xe2x80x9cApplied Physics Lettersxe2x80x9d, Vol. 65 (1994), pages 2922 to 2924 discloses covering the OLED on a glass substrate with a glass plate and adhesively bonding said plate to the glass substrate; bonding can be carried out using a commercial epoxy adhesive. The use of an epoxide for sealing organic light-emitting devices is also known from the U.S. Pat. Nos. 5,703,394 and 5,747,363.
DE 196 03 746 A1 discloses an electroluminescent device (with light-emitting organic material) whose capsuling comprises a multilayer system. This multilayer system is composed of at least one layer of plastic, and a metallic layer. In addition, there may be what is called a getter layer, which is embedded between two plastic layers. The plastic layers may consist, inter alia, of an epoxy resin.
In investigations on organic light-emitting diodesxe2x80x94for example, in storage tests at a temperature of 85xc2x0 C. and a relative humidity of 85%xe2x80x94it nevertheless proved impossible to find any commercially available adhesive which neither corroded the base metals used as cathode material nor impaired the light-emitting materials.
It is an object of an embodiment of the invention, therefore, to specify a method of encapsulating components based on organic semiconductors which on the one hand permits hermetic capsuling, so that harmful environmental effects are ruled out, and on the other hand does not lead to damage to the materials used in the components.
An object can be achieved in accordance with an embodiment of the invention by carrying out adhesive bonding using a UV-curable reactive adhesive comprising the following components:
an epoxy resin,
a hydroxy-functional reaction product of an epoxide compound with a phenolic compound,
a silane-type adhesion promoter, and
a photoinitiator, and also
if desired, filler.
The stated technical problem can be solved in accordance with an embodiment of the invention by joining the given parts by using a specially formulated adhesive based on epoxy resin. The given parts comprise a housing and a substrate which is the location for the organic semiconductor and the other materials needed for the functioning of the component: for example, metals as conductor tracks. Glass substrates in particular can be used, although plastic film substrates or plastic/glass composites may also be used. The housing may be a plate, particularly a glass plate, and is preferably a cover, made in particular from glass, although it may also be of ceramic or metal.
The adhesive used in accordance with an embodiment of the invention for constructive adhesive sealing, which can be applied without great effort, can be advantageously liquid or low-melting and therefore wetting. It is therefore particularly suitable for capillary gluing, i.e., for filling thin joints, but may alternatively be used as a casting resin or coating material.
The use of the adhesive of an embodiment of the invention produces the following advantages, including:
cost effective, automatable process;
rapid process;
low temperature process, thus no damage to the organic semiconductor;
no cumbersome insulation of adjacent conductor tracks which pass through the adhesion zone, owing to the electrically insulating nature of the adhesive.
Special advantages of the adhesives include:
one-component system, storable at room temperature for at least 1 year;
no outgassing of volatile constituents damaging to the functioning of the component;
high adhesion to glass over a wide temperature range;
flexibility sufficient over a wide temperature range, thereby minimizing mechanical stress as a result of thermal loading;
optical transparency;
higher O2 and H2O diffusion barrier;
electrically insulating activity;
ability to be processed under inert conditions;
rapid curing by UV irradiation.
The specific formulation of the adhesive gives it, in particular, high compatibility with components and also an effective air and moisture barrier effect. Accordingly, this adhesive is outstandingly suitable for capsuling (encapsulating) components based on organic semiconductors, especially light-emitting diodes.
When using covers which are bonded only at the edges, rather than plates, it is possible to avoid flat bonding and, consequent thereon, additional mechanical stress on the component.
The adhesive can be applied in particular, by capillary gluing, thus producing an extremely small adhesive joint of from 1 to 30 xcexcm, generally  less than 10 xcexcm. The result is a minimal area of attack for oxygen and water. As a result of the specific cationically initiated curing of the adhesive, i.e., the UV cure, xe2x80x9conexe2x80x9d polymer molecule is obtained, so to speak, which is free of low molecular mass, outgassable constituents harmful to the performance of the diodes. Moreover, it is possible to apply the adhesive in an inert gas atmosphere, which is necessary for the handling of the air-sensitive metal electrode materials, such as calcium. As a consequence of the UV-curability, a rapid process with a one-component resin is made possible. Encapsulation may also take place in addition to flat inorganic protective layers that are present on the components; such protective layers include, for example, SiO2 or Si3N4.
UV curing of the reactive adhesive may be followed advantageously by a thermal treatment, preferably up to a temperature of 120xc2x0 C. The lower temperature limit is generally about 10 to 20xc2x0 C. below the glass transition temperature of the cured adhesive. As a result of the thermal treatment (aftercure), the barrier effect of the adhesive film toward water and oxygen can be increased further.
The epoxy resin in the reactive adhesive may be an aliphatic, cycloaliphatic or aromatic epoxide, with preference being given to aliphatic and cycloaliphatic epoxides, i.e., ring-epoxidized, epoxides. The epoxy resin is composed advantageously of xe2x89xa770% by mass of an aliphatic and/or cycloaliphatic epoxide.
The aliphatic epoxide used can includes, in particular, epoxidized polybutadiene or epoxidized soybean oil; further epoxides which can be used are, for example, dodecene oxide and diglycidyl esters of hexahydrophthalic acid. The cycloaliphatic epoxides are preferably diepoxides. Examples of such diepoxides include 3,4-epoxycyclohexylmethyl 3xe2x80x2,4xe2x80x2-epoxycyclohexanecarboxylate (EEC) and bis(3,4-epoxycyclohexylmethyl)adipate. Further epoxides of this kind are, for example, compounds in which the aliphatic chain of the adipic acid derivative, composed of 4 methylene units, is replaced by a chain with from 5 to 15 methylene units.
The reactive adhesive can advantageously contain from 3 to 80% by mass of hydroxy-functional reaction product, which serves as adhesion-promoting component. Compounds of this kind, which are known per se (DE 197 51 738 A1), can be prepared by base-catalyzed reaction of an epoxide compound with a phenolic compound, the basic catalyst preferably being an onium hydroxide, such as tetramethylammonium hydroxide: 
The radical R1 here may be unreactive, as is the case, for example, with an alkyl or ester group, but may also be epoxy-functional. The radical R2 may carry further phenolic groups, leading to the formation of reaction products of higher molecular mass; for each phenolic OH group, one aliphatic OH group is formed.
The hydroxy-functional reaction products, which advantageously possess a molar mass xe2x89xa7500, may further contain epoxide groups, i.e., may also be epoxy-functional. This is the case when the radical R1 carries an epoxide group. Compounds of this kind can be prepared by reacting bifunctional epoxides with bisphenols in a molar ratio of from 2:1 to 20:1; for example, by reacting bisphenol A with bis(3,4-epoxycyclohexylmethyl)adipate: 
The tetraethylammonium hydroxide catalyst used herexe2x80x94like other onium hydroxides as wellxe2x80x94is thermolabile and after the end of the reaction breaks down into volatile and harmless products. The hydroxy-functional reaction products therefore contain no substances which might adversely affect the cationic curing of the reactive adhesive.
The silane-based adhesion promoters are, in particular, alkoxy-functional silanes, generally methoxy- and ethoxy-functional silanes. The silanes normally carry at least one further group attached by way of an Sixe2x80x94CC bond, as is the case, for example, with glycidyloxy-propyltrimethoxysilane. The adhesion promoter is added to the reactive adhesive in an amount of preferably from 0.05 to 2% by mass.
The photoinitiator is advantageously an onium salt, especially a triarylsulfonium salt with hexafluorophosphate, hexafluoroarsenate or hexafluoroantimonate as the anion, such as tri-phenylsulfonium hexafluoroantimonate. The reactive adhesive preferably has a photoinitiator content of from 0.01 to 5% by mass.
The reactive adhesive may also comprise filler. Suitable fillers include very finely ground minerals, particularly those based on silica, examples being quartz flours or fused silica flours, and finely dispersed silicas. These fillers are used in order to lower the thermal expansion coefficient of the adhesive. It may therefore be of importance for the adhesive to tolerate high fractions of filler while retaining good flow properties. Up to 70% by mass of filler may be contained within the reactive adhesive. The filler may also serve for reducing the moisture uptake or diffusion and for reducing shrinkage.
Besides the filler, further conventional additions or additives may be present, such as dyes, pigments, wetting auxiliaries, flow assistants, adhesion promoters, thixotropic agents, defoamers, flow modifiers, stabilizers, and flame retardants. By means of these substances it is possible to give the reactive adhesive additional properties, such as color, special rheological properties, and low flammability.
Advantageously, the reactive adhesive may further comprise a polyol; these are, in particular, polyesterpolyols and polyetherpolyols. Examples are polyestertriols based on trimethylolpropane and xcex5-caprolactam, and also polyesterdiols based on ethylene glycol. Polyetherpolyols are available in large numbers; chain extension here takes place, for example, with ethylene oxide or propylene oxide. Depending on their molar mass and OH content, the polyols, which serve to modify the mechanical properties of the cured adhesive, are used in an amount such that there is no excess of OH groups over epoxide groups.
The reactive adhesive may further comprise, in addition, a surface-active compound, in particular a surface-active siloxane; additives of this kind serve as defoamers and flow assistants. The fraction of the surface-active compound is low, generally just 0.1 to 0.5% by mass.